HELP sick goldfish - need advice

Hi! I have a sick goldfish and I can't figure out what to do.

I am a teacher at a preschool. This goldfish was here before I was hired and this year I have taken responsibility for them in the classroom. I've been trying to do research and given him a good home, not knowing how to take care of fish or how much work and care they really need. I am trying to learn more but there are definitely areas where I'm falling short.

His name is Walter, he is a comet/common goldfish. He has been sick from ammonia before and I have treated it. As of a week ago, he had fluffy red/white spots on his side that had faded to now be bumps and they're hard to see. He also has white spots on his tail. I think it is a mixture of ich and a fungal infection.

He is now in a 20 g hospital tank (away from our other goldfish) and has been treated with Super Ick Cure since last Saturday and Pimafix since this Wednesday. I took the filter out since the bottle said no activated carbon and put a bubbler in instead, as well as a heater that brought the temp to 78 F. I did a water change with water conditioner and aquarium salt Wednesday. I tried to feed him and he spit out the food, then I scooped all the food back out.

Other symptoms included lethargy, swimming eratically, bumping into things, rubbing against things, rapid breathing, and loss of appetite. As of this morning, I think he has tail rot and is gasping at the surface.

What else can I do? I can include pictures if needed and am driving up to a fish shop for supplies and advice this weekend.